This proposal outlines two approaches to some central problems which must be overcome before we will understand eukaryotic gene-control processes in any detail. The first is the relation between structure and function in the fundamental unit of chromatin, the nucleosome. We have proposed a possible relationship between gene activation, histone modification, and an "open" (vs. closed) nucleosome. Moreover, regulatory processes in eukaryotes appear to involve a "spreading effect", in which modified nucleosomes induce structural changes in their neighbors. If this is correct, non-histone regulatory proteins such as steroid receptors must determine what regions of the genome are to change in this way. We propose to develop a purified in vitro system in which the interaction of such regulatory proteins with nucleosomes and histone-modifying enzymes can be examined in detail. Histone acetylases will be purified to homogeneity and their specificity characterized. Likewise the 5S estradiol receptor (and possibly other more predominant effector proteins) will be purified. With these components, well-defined chromatins will be examined for biologically-relevant nucleosome modifications induced synergistically, with or without additional added DNA-binding components. The second approach involves a search for the chemical responsible for ZPA activity in the embryonic chick limb bud. This "morphogen" may well be analogous to the hormones which interact with steroid receptors, in so far as information with respect to its local concentration must be accurately relayed to the genome. Thus, one might hope to eventually detect the receptor protein(s) in this system, which should then lead us directly to the crucial, developmentally relevant genes it must control.